The airfoils of a gas turbine engine, as well as other engine component parts, are subject to vibrational stress that can adversely affect the lifetime thereof as well as overall engine efficiency. The gas turbine industry, in consideration of this stress, has developed numerous ways of restraining or damping airfoil vibrations. One approach that has been taken involves frictionally damping particular modes of airfoil vibrations by interlocking the tips of tip-shrouded blades. To create the interlock, the blades are twisted during assembly of the engine. The applied twist is in a circumferential direction as viewed along the longitudinal axis of each blade. Such an assembly technique is taught in U.S. Pat. No. 3,185,441 to Reuter. It is now, in the art to configure the interlock between adjacent tip shrouds substantially as a "Z" thereby providing frictional damping only along the diagonals of the "Z" rather than along the entire lengths of the adjacent tip shroud edges.
Twisting the blades creates a load on the tip shrouds that is normal to the plane of contact between adjacent shrouds. This particular load, hereafter referred to as an elastic load, arises from the inherent tendency of the blade to rebound to its former shape. In other words, the blade tries to untwist.
The elastic load causes the tip shrouds to rub against each other during engine operation and thereby dissipates the airfoil vibrations at least in part. The elastic load may be insufficient in and of itself, however, to provide the total desired blade damping.
Another source of tip shroud load is available for rotating blades, however. During engine operation the centrifugal forces generated by the rotation will create an additional load on the tip shrouds. In other words, as the centrifugal forces act on the blade, the blade tries to lengthen and does so by trying to untwist. Thus, this rotationally generated, or centrifugal, load also lies normal to the plane of contact between the tip shrouds and adds to the load created by the inherently elastic blade material trying to return to its former, untwisted shape.
The total tip shroud load, centrifugal and elastic, in many cases, may be sufficient to provide the desired level of blade damping. In such cases the centrifugal and elastic loads may be considered to be inversely proportional to each other. That is, for a given constant total load, as the centrifugal load is increased, the elastic load may correspondingly be decreased. The faster the blade rotates, in other words, the greater the centrifugal load is and the less the elastic load need be. Conversely, the slower the blade rotates, the smaller the centrifugal load is and the greater the elastic load must be to maintain the constant total load. Typically, then, as an airfoil rotates more and more slowly at normal operating conditions, it must be subjected to ever increasing degrees of assembly twist to provide the desired damping characteristics.
With many features of gas turbine engine technology trade offs exist with many design features--i.e., solving one problem may create others with the solution--and so it is with twisting blades during assembly. Twisting the blades can introduce high stress levels in certain localized regions of the airfoil lying adjacent the tip shroud and the blade root. These areas of high stress are subject to the development of cracks during engine operation and thus create a potential failure mode for the blade. In other words, the need to twist the blades to create the load necessary for proper damping of the blade during engine operation stresses the blade material structure so that cracks can develop more quickly than they would without the twist. In turn, the blade cracking can lead to blade failure with the resultant loss of the blade itself as well as downstream blades which are struck by portions of the disintegrating blade.
The degree of twist is directly related to the amount of resulting stress. Thus, the greater the twist is, the greater the resulting stress will be. Where the degree of twist is great due to the need to compensate for the loss of the centrifugal load due to a slowly rotating engine, the stress will correspondingly be greater as will the potential for failure of the blade itself and for other engine components. It would be desirable, therefore, to provide a blade having a reduced failure mode due to high local stresses introduced by twisting of the blade during engine assembly while still retaining the damping ability provided by interlocking tip shrouds.